The present invention relates to receptor pad structures for chip carriers, and in particular, non-floating BLM C-4 receptor pads for organic chip carriers preferably utilizing eutectic solder.
Basically, the chip carriers employ eutectic solder consisting of either a tin/lead plated solder, or it is adapted to be solder-injected or flip chip screen printed on the chip carrier C4 (controlled collapsed chip connect) receptor pads. In essence, the eutectic solder facilitates the use of existing C4 technology in conjunction with organic chip carriers, and wherein C4 bumps consists of Ball Limiting Metallurgy (BLM) chrome and chrome copper phased and evaporated 3.5% by weight of tin and 96.5% by weight of lead. In order to be able to produce solder joints between the eutectic solder on the chip carrier and the C4 bump, it is ordinarily the practice to deposit a controlled amount of eutectic solder on the C4 receptor pad. Basically, this amount of solder deposition is empirically determined and is predicated on various parameters including: chip size and the number of C4 bumps; the chip pitch; the C4 ball diameter; the C4 receptor pad area (normally determined by the copper trace width of the circuit lines and solder mask opening); and, finally, the encountered organic chip carrier warpage, which may be predicated on temperature differentials between the chip carrier surfaces. Ordinarily, each one of these of parameters, in turn, is derived through a set of specified secondary parameters, and upon all of the parameters being optimized, the eutectic solder is deposited on the organic chip carrier so as to form high reliable solder joints. In contrast therewith, when the above-referenced parameters are not properly optimized, the eutectic solder creeps up to the BLM (i.e. the C4 bump) and detaches the latter from the chip. Upon this occurrence, the solder joint will fail to maintain or lose its reliability, and the system will fail.
Currently, organic chip carrier C4 receptor pads create floating BLM which, occasionally, even in the instances of deposition of equal amounts of solder on various pads configurations, result in the obtaining of various or differing bump heights. Thus, the solder volume distributes itself over the pad area, and inasmuch as the pad areas are frequently of different sizes and shapes, this results in the creation of unequal bump heights. Consequently, even though the solder bumps are flattened prior to the placement of the chips on the printed circuit board, the tall bumps will rise faster which creates chip skewing or tilting, thereby adversely affecting the integrity and functional reliability of the formed solder joints. Moreover, as the chip settles into the molten solder during solder reflow, the tall bump solder will creep around the C4 bump and can easily reach the BLM, resulting in the adversely influencing BLM floating. Current technology utilizing the openings or windows of various insulating material masks with regard to the formation of solder joints have failed to solve the above encountered problem in ensuring the reliability of the solder joints. In essence, the current state-of-the-technology, has not been found an effective or fully satisfactory method of obviating the problems encountered in attempting to provide uniform and controlled formations of solder joints with regard to receptor pad structures.